1. Field of the Invention
The present invention relates to superconducting bearings, and more particularly to superconducting bearings for force-measuring instruments such as gravimeters useful in borehole and surface surveys.
2. The Prior Art
Gravimeters have long been used for land, sea, and air surveys of the earth's gravitational field. These surveys have been conducted for geophysical research purposes, and to locate mineral or hydrocarbon deposits beneath the surface of the earth.
Borehole gravity measurements are used commercially for reservoir delineation. The difference between the measured magnitude of gravitational force at two stations in a borehole, divided by the station spacing, is directly proportional to the density of a disc whose upper and lower surfaces are defined by the station locations. The radius of the disc is roughly five times the station spacing; 90% of the signal originates from within this radius. Station spacings of one to ten meters are common, although any spacing can be used. The measurement is thus significantly deeper than that of other borehole measurement instruments, but has the disadvantage that the gravimeter cannot be moved during a measurement. A precision of 0.01 g/cc in the density determination is required to satisfy the demands of the oil exploration industry. This requires that the force of gravity be measured to better than one part per billion.